The present invention relates to a novel class of photooxidizable compounds. These compounds are complexes of xanthene dyes such as Rose Bengal, Eosin, Erythrosin, Fluorescein and esters thereof and gegen ions such as sulfonium, iodonium, phosphonium, pyrylium, hydrazinium, and hydrazide ions. These compounds are useful as free radical initiators or as photobleachable dyes for use in photobleach imaging systems. They may also be useful as catalysts in thick layer photopolymerization processes which produce layers of photopolymer up to 75 microns or more thick. Thus "in depth" photographic processes may be possible with these compounds.
An excellent summary of dye-sensitized photopolymerizable compositions is provided by Eaton, "Dye Sensitized Photopolymerization," Advances in Photochem., Vol. 13, pp. 427-87. These compositions include a photoreducible or photooxidizable dye and a reducing agent. Upon exposure, actinic radiation is absorbed by the dye and the dye is raised to its excited electronic state, the lifetime of which may be less than 10.sup.-9 and up to 10.sup.-3 second, depending upon whether it is a singlet or triplet state. During this time, absorbed energy in the form of an electron must be transferred from the reducing agent to the photoreducible dye molecule or from the photooxidizable dye molecule to the oxidizing agent to produce the free radical. Prior initiator systems tend to provide relatively low film speeds. One reason for this is that the rate of electron transfer is diffusion controlled. Another is that most prior systems involve photoreduction of the dye triplet which is substantially slower than other reactions which would proceed through the dye singlet. The excited dye must interact (collide) with another molecule in the composition which quenches the dye and generates a free radical. This limits the film speed which one can achieve.